Uterine fibroids are benign (noncancerous) tumors of muscle tissue in the uterus. They are also called myomas or leiomyomas. Fibroids occur when a single muscle cell in the wall of the uterus multiplies and grows to form a noncancerous tumor. Fibroids can change the shape or size of the uterus and sometimes the cervix (lower part of the uterus). Women usually have more than one fibroid tumor but single fibroids are possible. Whether fibroids cause symptoms or require treatment depends on their location, size, and number.

Fibroids are usually found in or around the body of the uterus, but sometimes are in the cervix. There are three main types of fibroids based on where they are found:

Subserosal are in the outer wall of the uterus (55%)

Intramural are found in the muscular layers of the uterine wall (40%)

Submucosal protrude into the uterine cavity (5%)

Fibroids can also be connected to the uterus by a stalk (pedunculated), or attached to nearby ligaments or organs, such as the bladder and bowel. Fibroids are rarely found outside the pelvic cavity.

For pregnancy to occur, every step of the human reproduction process has to happen correctly. The steps in this process are:

One of the two ovaries releases a mature egg.

The egg is picked up by the fallopian tube.

Sperm swim up the cervix, through the uterus and into the fallopian tube to reach the egg for fertilization.

The fertilized egg travels down the fallopian tube to the uterus.

The fertilized egg attaches (implants) to the inside of the uterus and grows.

In women, a number of factors can disrupt this process at any step. Female infertility is caused by one or more of the factors below.

Ovulation disorders

Ovulating infrequently or not at all accounts for most cases of infertility. Problems with the regulation of reproductive hormones by the hypothalamus or the pituitary gland or problems in the ovary can cause ovulation disorders.

Polycystic ovary syndrome (PCOS).PCOS causes a hormone imbalance, which affects ovulation. PCOS is associated with insulin resistance and obesity, abnormal hair growth on the face or body, and acne. It’s the most common cause of female infertility.

Hypothalamic dysfunction.Two hormones produced by the pituitary gland are responsible for stimulating ovulation each month — follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Excess physical or emotional stress, a very high or very low body weight, or a recent substantial weight gain or loss can disrupt production of these hormones and affect ovulation. Irregular or absent periods are the most common signs.

Primary ovarian insufficiency.Also called premature ovarian failure, this is usually caused by an autoimmune response or by premature loss of eggs from your ovary, possibly as a result of genetics or chemotherapy. The ovary no longer produces eggs, and it lowers estrogen production in women under age 40.

Too much prolactin.The pituitary gland can cause excess production of prolactin (hyperprolactinemia), which reduces estrogen production and can cause infertility. This can also be caused by medications you’re taking for another condition.

Damage to fallopian tubes (tubal infertility)

Damaged or blocked fallopian tubes keep sperm from getting to the egg or block the passage of the fertilized egg into the uterus. Causes of fallopian tube damage or blockage can include:

Pelvic inflammatory disease, an infection of the uterus and fallopian tubes due to chlamydia, gonorrhea or other sexually transmitted infections

Previous surgery in the abdomen or pelvis, including surgery for ectopic pregnancy, in which a fertilized egg implants and develops somewhere other than the uterus, usually in a fallopian tube

Endometriosis

Endometriosis occurs when tissue that typically grows in the uterus implants and grows in other places. This extra tissue growth — and the surgical removal of it — can cause scarring, which can block fallopian tubes and keep an egg and sperm from uniting.

Endometriosis can also disrupt implantation of the fertilized egg. The condition also seems to affect fertility in less-direct ways, such as damage to the sperm or egg.

Uterine or cervical causes

Several uterine or cervical causes can interfere with the egg implanting or increase the risk of miscarriage:

Benign polyps or tumors (fibroids or myomas) are common in the uterus. Some can block fallopian tubes or interfere with implantation, affecting fertility. However, many women who have fibroids or polyps do become pregnant.

Problems with the uterus present from birth, such as an unusually shaped uterus, can cause problems becoming or remaining pregnant.

Cervical stenosis, a narrowing of the cervix, can be caused by an inherited malformation or damage to the cervix.

Sometimes the cervix can’t produce the best type of mucus to allow the sperm to travel through the cervix into the uterus